The present invention relates generally to the operation of an oxygen electrode for use in an electrolytic cell and particularly for the production of chlorine and caustic (sodium hydroxide) in such a manner as to significantly reduce the voltages necessary for the operation of such electrolytic cells and to increase substantially the power efficiencies available from such electrolytic cells utilizing oxygen electrodes. More particularly, the present disclosure relates to improved methods of operation of oxygen electrodes which include utilizing a positive air to liquid pressure drop on the air feed side of the oxygen electrode to improve performance, control of the total flow of the gas feed stream to improve the mass transfer within the air feed side of the oxygen electrode at the reaction sites, humidification of the air feed to the oxygen electrodes to reduce the drying out and delamination of the oxygen electrode so that it might function at a higher current density over a longer lifetime, and the elimination of certain gases such as carbon dioxide from the air feed before feeding to increase the lifetime of the oxygen electrodes by elimination of salts which might be formed upon the porous structure of the oxygen electrode during the use thereof. These methods of operation may be utilized singularly or preferably in combination to produce higher power efficiencies at lower voltages so as to produce a more energy-efficient oxygen electrode in an electrolytic cell especially suitable for the production of chlorine and caustic (sodium hydroxide).
Chlorine and caustic are essential large volume commodities which are basic chemicals required by all industrial societies. They are produced almost entirely electrolytically from aqueous solutions of alkaline metal halides or more particularly sodium chloride with a major portion of such production coming from diaphragm-type electrolytic cells. In the diaphragm electrolytic cell process, brine (sodium chlorine solution) is fed continuously to the anode compartment to flow through a diaphragm usually made of asbestos particles formed over a cathode structure of a foraminous nature. To minimize back migration of the hydroxide ions, the flow rate is always maintained in excess of the conversion rate so that the resulting catholyte solution has unused or unreacted sodium chloride present. The hydrogen ions are discharged from the solution at the cathode in the form of hydrogen gas. The catholyte solution containing caustic soda (sodium hydroxide), unreacted sodium chloride and other impurities, must then be concentrated and purified to obtain a marketable sodium hydroxide commodity and sodium chloride which is to be reused in electrolytic cells for further production of sodium hydroxide and chlorine. The evolution of the hydrogen gas utilizes a higher voltage so as to reduce the power efficiency possible from such an electrolytic cell thus creating an energy inefficient means for producing sodium hydroxide and chlorine gas.
With the advent of technological advances such as dimensionally stable anodes and various coating compositions therefor which permit ever narrowing gaps between the electrodes, the electrolytic cell has become more efficient in that the power efficiency is greatly enhanced by the use of these dimensionally stable anodes. Also, the hydraulically impermeable membrane has added a great deal to the use of the electrolytic cells in terms of selective migration of various ions across the membrane so as to exclude contaminates from the resultant product thereby eliminating some of the costly purification and concentration steps of processing. Thus, with the great advancements that have tended in the past to improve the efficiency of the anodic side and the membrane or separator portion of the electrolytic cells, more attention is now being directed to the cathodic side of the electrolytic cell in an effort to improve the power efficiency of the cathodes to be utilized in the electrolytic cells thus create a significant energy savings in the resultant production of chlorine and caustic. Looking more specifically at the problem of the cathodic side of a conventional chlorine and caustic cell, it may be seen that in a cell employing a conventional anode and a cathode and a diaphragm therebetween, the electrolytic reaction at the cathode may be represented as EQU 2H.sub.2 O+2e.sup.- yields H.sub.2 +2OH.sup.-
The potential of this reaction versus a standard H.sub.2 electrode is -0.83 volts. The desired reaction under ideal circumstances to be promoted at the cathode would be EQU 2H.sub.2 O+O.sub.2 +4e.sup.- yields 4OH.sup.-
The potential for this reaction is +0.40 volt which would result in a theoretical voltage savings of 1.23 volts. The electrical energy necessarily consumed to produce the hydrogen gas which is an undesirable reaction of the cathode of the conventional electrolytic cells has not been counterbalanced efficiently in the industry by the utilization of the resultant hydrogen since it is basically an undesired product of the reaction. While some uses have been made of the excess hydrogen gas, those uses have not made up the difference in the expenditure of electrical energy necessary to evolve the hydrogen thus, if the evolution of a hydrogen could be eliminated, it would save electrical energy and thus, make production of chlorine and caustic a more energy-efficient reaction.
The oxygen electrode presents one possibility of elimination of this reaction since it consumes electrochemically activated oxygen to combine with water and the electrons available at the cathode in accordance with the following equation EQU 2H.sub.2 O+O.sub.2 +4e.sup.- yields 4OH.sup.-
It is readily apparent that this reaction is more energy efficient by the very absence of the production of any hydrogen at the cathode, and the reduction in potential as shown above. This is accomplished by feeding an oxygen rich fluid such as air or oxygen to an oxygen side of an oxygen electrode where the oxygen has ready access to the electrolytic surface so as to be consumed in the fashion according to the equation above. This does, however, require a slightly different structure for the electrolytic cell itself so as to provide for an oxygen compartment on the cathodic side of the cathode so that the oxygen rich substance may be fed thereto.
The oxygen electrode itself is well-known in the art since the many NASA projects utilized to promote space travel during the 1960s also provided funds for the development of a fuel cell utilizing an oxygen electrode and a hydrogen anode such that the gas feeding of hydrogen and oxygen would produce an electrical current for utilization in a space craft. While this major government-financed research effort produced many fuel cell components including an oxygen electrode, the circumstances and the environment in which the oxygen electrode was to function were quite different from that which would be experienced in a chlor-alkali cell. Thus, while much of the technology gained during the NASA projects is of value in the chlor-alkali industry with regard to development of an oxygen electrode, much further development is necessary to adapt the oxygen electrode to the chlor-alkali cell environment.
Some attention has been given to the use of an oxygen electrode in a chlor-alkali cell so as to increase the efficiency in the manner described to be theoretically feasible, but thus far the oxygen electrode has failed to receive significant interest so as to produce a commercially effective or economically viable electrode for use in an electrolytic cell to produce chlorine and caustic. While it is recognized that a proper oxygen electrode will be necessary to realize the theoretical efficiencies to be derived therefrom, the chlor-alkali cell will require operational methodology significantly different from that of a fuel cell since an electrical potential will be applied to the chlor-alkali cell for the production of chlorine and caustic in addition to the supply of an oxygen rich fluid to enhance the electrochemical reaction to be promoted. Therefore, it would be advantageous to develop the methodology for the operation of an oxygen electrode directed specifically toward the maximization of the theoretical electrical efficiencies possible with such an oxygen electrode in a chlor-alkali electrolytic cell for the production of chlorine and caustic.